1. Field of the Invention
The present invention generally relates to a method of preparing thin film gas sensors. It more particularly relates to a gas-sensing layer of SnO2 thin-film for stable detection of reducing gases. It also relates particularly to thin film CuO catalytic layer for the detection of dilute sulfur compound gases.
2. Description of Prior Art
Various forms of SnO2, including sintered block, thick film, and thin film, have widely been researched and used as gas sensors for hydrogen, hydrocarbons, carbon monoxide, and nitrous oxides. Recently, thin film gas sensors attracted a lot of industrial and scientific attention due to their high sensitivity and possibility of integration. In producing SnO2 thin films, it is important to avoid formation of amorphous and intermediate phases such as Sn or SnO, which may adversely affect the sensor characteristics.
Takeyuki Suzuki et al. deposited SnO2-x thin films by sputtering SnO2 target with argon ion beam (Journal of Materials Science 23, 1988, 145-149, Journal of Materials Science 23, 1988, 1106-1111). In this report, the substrate temperature was below 100xc2x0 C., and the resulting films were amorphous and nonstoichiometric. Therefore, an intermediate phase of Sn3O4 was formed after a heat treatment at 300xc2x0 C., and the hydrogen sensitivity of the sensor was very low. Wan-Young Chung et al. deposited SnO2 thin films on Si wafers through thermal evaporation and subsequent thermal oxidation of Sn films (Sensors and Actuators B, 20, 1994, 139-143). The research reported detachment of the film from wafers after thermal oxidation, which was caused by large volume expansion during oxidation and phase transformation. Seok-Keun Koh et al. used reactive ionized cluster deposition to produce SnO2 films at room temperature (Japan Patent 9-170993). The oxygen ion beam used in the patent produced amorphous and nonstoichiometric films at accelerating voltage as high as 4 kV.
In the case of CuO with p-type conductivity, it has been known that its addition to SnO2 greatly increases the sensitivity and selectivity to hydrogen sulfide (T. Maekawa et al., Chem Lett., 1991 (1991) 575, J. Tamaki et al., Sensors and Actuators, B9 (1992) 197, M. Rumyantseva et al., J. Mater. Chem., 1997, 7(9) 1785-1790). This type of sensor is reported to have stable and excellent sensing characteristics to hydrogen sulfide in the concentration range between 10 to 100 ppm at 200xc2x0 C.
The excellent sensing characteristics of CuO-added SnO2 to hydrogen sulfide is ascribed to p-n function formed between n-type SnO2 and p-type CuO. The p-n junction builds very high electrical resistance across the interface by forming a thick charge depletion layer on each side. When CuO is exposed to sulfur compound gases such as hydrogen sulfide, it is converted to CuS with metallic conductivity through chemical reactions as in the example shown below:
CuO+H2Sxe2x86x92CuS+H2O 
CuO+CH3SH+3/2 O2xe2x86x92CuS+CuO+2H2O 
Consequently, the p-n junction as well as the charge depletion layer shall disappear causing the resistance of the sensor to drop dramatically.
The sensors described above performed excellent sensing characteristics to hydrogen sulfide at a concentration higher than 10 ppm, but was very poor at a lower concentration. To solve this problem, various attempts have been made, such as fine distribution of CuO through chemical fixation method or thin film deposition with spin coating or vacuum evaporation. These attempts succeeded in lowering the detection range to a few ppm or to 0.1xcx9c0.3 ppm, but the sensitivity is still less than around 10. Kazuhisa Hasumi et al. produced CuOxe2x80x94SnO2 type hydrogen sulfide sensor through thick film technology and spin coating (U.S. Pat. No. 5,618,496). J. Tamaki et al. produced gas sensors with sensitivity of 8.8 to hydrogen sulfide of 0.2 ppm through evaporation and thermal oxidation of Sn and Cu (Sensors and Actuators, B 49 (1998) 121-125). This sensor is reported to show sensitivity to hydrogen sulfide of 0.02 ppm, but the value was quite low, and the response time was as long as 10 minutes.
It is, therefore, an object of the present invention to provide stoichiometric, dense, highly electrically resistant and crystalline SnO2 thin films at low substrate temperature showing excellent sensing characteristics to reducing gases. It is also an object of the present invention to provide stoichiometric, dense and crystalline CuO thin films capable of detecting dilute sulfur compound gases.
It is a further object to provide the process by which stoichiometric, dense, highly resistant and crystalline thin film SnO2 gas sensing layer of the present invention to be made. It is also a further object to provide the process by which stoichiometric, dense and crystalline thin film CuO catalytic layer of the present invention to be made.